1. Field of the Invention
The present invention relates to a cell switch module, a transmitter and an active/standby switching method in such a transmitter, and more particularly to switching executed at the time of up-grading a network.
2. Description of the Related Art
In the known public transmission network, audio signals and data information are transmitted to be processed via an infrastructure of SONET (Synchronous Optical Network) in a format of STM (Synchronous Transfer Mode). Such service signals according to the STM format are accessed, in each transmitter, at an STS-1 (Synchronous Transport Signal: 51.84 MHz) level conforming with the SONET frame format or a VT (Virtual Tributary: 1.726 MHz) level. External interfaces employed therefor conform with OC-48 (optical signal: 2.4 GHz), OC-12 (optical signal: 600 MHz), OC-3 (optical signal: 150 MHz), DS3 (electric signal: 44.736 MHz), DS1 (electric signal: 1.544 MHz) and so forth. Further, recently, there is practically used of a data information service which is based on the ATM (Asynchronous Transfer Mode) signal format to realize effective utilization of transmission lines and the band of transmission equipment.
In laying an optical fiber network for use in SONET or the like, the construction thereof by the network service contractor concerned is carried out on the basis of a long-term plan since the construction costs much and the required term for construction of such fiber network is long. On the other hand, it is expected that demands for digital data services using optical fiber networks as transmission lines will increase rapidly after initial diffusion, as represented currently by the Internet. In order to comply with the circumstances thus changing so fast, it is considered necessary to develop an improved transmitter which has a high-speed optical line terminator with a great transmission capacity per optical fiber and a fast digital line terminator adapted for transmitting, via the same transmission channel, both the conventional telephone service where the demand of transmission information is predictable with relative facility and the aforementioned digital data service, and is capable of freely changing, in accordance with needs of such services, the ratio of using such services (e.g., by increaseing the band of ATM cells while correspondingly decreasing the band of data signals other than ATM cells, such as DS1 TDM or DS3 TDM).
Since some modification or extension of such service structure is carried out after practical running, it is necessary to up-grade the same in a manner to render the harmful influence least on the service network being currently used. For example, loss of ATM cells needs to be minimized at the time of any modification or extension. However, none of the known transmitters adapted for handling both STM and ATM signals is equipped with an ATM cell switching function and another function of freely changing the band of ATM cells and that of other signal, although both STM and ATM signals can be transferred via the same optical fiber.